1. Field of the Invention
The present invention relates to absorbent structures. Specifically, the present invention relates to an absorbent structure comprising a nonwoven masking layer and a nonwoven distribution layer.
2. Description of the Related Art
A variety of absorbent structures are known for use in disposable absorbent products intended to absorb body fluids. Examples of such products include diapers, feminine care products, training pants, adult incontinence products and the like. In general, one of the most economical, liquid-absorbent materials for use in such products is cellulosic fibers, such as comminuted wood pulp fluff. While being absorbent, undensified wood pulp fluff typically does not transport or distribute a liquid particularly well. Thus, liquid absorbed by undensified wood pulp fluff tends to remain in the fluff at the point where it is initially absorbed.
Various solutions have been proposed to allow wood pulp fluff to better distribute an absorbed liquid. For example, U.S. Pat. No. 4,213,459 to Sigl describes decreasing the pore size of an absorbent material along the length of an absorbent product. The improved capillary action of the pores overcomes the force of gravity and causes liquid to move along the length of the absorbent product. The reduction in pore size tends to reduce the total absorption capacity of the absorbent batt.
In an attempt to improve on the teachings of Sigl, U.S. Pat. No. 4,699,619, issued Oct. 13, 1987, to Bernardin, describes a two-layer structure for absorption of body fluids. The upper layer has a lower density and/or larger pore size than the lower layer. The upper layer serves as an acquisition layer to initially absorb a liquid at the point of liquid application. The absorbed liquid is then drawn into the lower layer, having a higher density or smaller pore size, and is transported throughout the lower layer so as to desorb the portion of the upper layer where liquid waste initially accumulated. After the lower layer distributes the absorbed liquid, the liquid is free to transfer from the lower layer back into the upper layer at points remote from the location of initial fluid insult. While such a two-layer approach has proven effective, densification of the lower layer tends to reduce the absorbent capacity of the lower layer.
The use of shaped fibers to form various woven and nonwoven products is known. For example, U.S. Pat. No. 4,129,679 issued Dec. 12, 1978, to Woodings describes regenerated cellulose filaments having a collapsed hollow structure and a multi-limbed cross section. The fibers are said to possess a high capability of water imbibition. The fibers can be formed into woven fabrics, like toweling, and non-woven fabrics and wadding, such as diapers, sanitary napkins, tampons and swabs. European Patent Application 0 301 874 published Feb. 1, 1989, is directed to cellulosic fibers having a decitex of less than 5.0 and a multi-limbed cross section. The limbs have a length-to-width aspect ratio of at least 2:1. The fibers can be formed into woven, nonwoven, or knitted fabrics and are described as being especially useful for absorbent products. Japanese Kokoku Patent No. SHO 62[1987]-53605 published Nov. 11, 1987, is directed to synthetic fibers having shaped cross sectional views meeting certain criteria. Again, the fibers are said to have excellent water absorptivity when formed into nonwoven products.
When nonwoven webs containing shaped fibers are employed in disposable absorbent products, the ability to absorb a liquid is generally not sufficient to ensure optimum performance. For example, during use, many disposable absorbent products are exposed to multiple insults of a liquid. In order to ensure proper absorption of subsequent insults, it is generally desired that the first insult of liquid not only be absorbed but also transported within the absorbent products to areas remote from the point of insult. European Patent Application 0 391 814 published Oct. 10, 1990, describes the use of shaped fibers in absorbent products to transport liquids. The shaped fibers are described as possessing a specific surface area relative to diameter.
When nonwoven webs containing shaped fibers are employed in disposable absorbent products, it is desirable that the web not only be able to transport a liquid but also be able to quickly accept a liquid. Additionally, it is often desirable to be able to control the direction in which liquid present in the product is transported. This is particularly true when the width of the absorbent product is relatively narrow compared to its length. The ability to control the direction of liquid transport allows prevention of liquid transferring out the side edges of a product.
Absorbent products, which are capable of good liquid transport, are not without disadvantages. Liquids to be absorbed by such absorbent products are often applied to the products in a relatively localized area. When the liquid is subsequently transported throughout the product, a larger surface of the product tends to become wet and discolored by the absorbed liquid. This may be both uncomfortable and unattractive.